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imac 27 820-00134 no 3v3_s5

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    #1

    imac 27 820-00134 no 3v3_s5

    hi all. got this board with bent cpu pins. it was not working and while trying new cpu and ram, customer bent many cpu pins.
    i have replaced cpu socket, but board didnt give first led.
    i have 12 volt, but it's missing pp3v3_s5. if i connect multimeter and give power i see a spike of 1,5 volt then dropping slowly. if i remove power after connecting it i see that spike, then when it reach like 0,10 volt it goes up to 1,5 volt again even if psu has been disconneted.

    already replaced u7600 isl6238 chip from another board (not sure if it's 100% working), but same thing happen.
    pin 12 voltage is 4,6 volt.

    volts on c5400 pin 1 (pp3v3_s5 ) goes to 0,15 volt when psu is connected.

    resistance on L7610 is high, like no short.

    so: 0 to 1,5 volt spike and going down slowly on L7610, but 0,15 volt on pp3v3_s5 line.
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    #2
    Re: imac 27 820-00134 no 3v3_s5

    Most iMac's I work on like that have a shorted switched rail after S5. Power supply starts and shuts down instantly as a result. I go through each switched rail and check for shorts (like PP3V3_S4 etc). Last one I think was a short on PP3V3_S0 and was a cap at the bottom of the board near the rear connectors.

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      #3
      Re: imac 27 820-00134 no 3v3_s5

      tnx for reply. ill check later

      edit: however the rail that goes on for a bit then of is a s5 rail ( pp3v3_s5) and imac first light is not even on...

      on pp3v3_s5 coil there is no short and that line goes on up to 1.5 volt then back to 0 volt
      Last edited by dellxps15; 01-30-2023, 11:17 AM.

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        #4
        Re: imac 27 820-00134 no 3v3_s5

        update:

        after some investigation today i found that u7600 was getting 3,7v in instead of 12volt, so i measured 1omh resistor r7603 and it was bad. after replacing it i got 2,6volt on L76120.

        after some more testing i removed q7690 and i got 3,4 volt on pp3v3_s5_ssd. after put it back and remove u7620 i got 3,12 volt on pp3v3_s5_ssd. now im waiting both component to be sure to get all new and working and after i got them in around 15-20 days from china, ill update the statement

        tnx for helping

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